The invention relates to a method for controlling a deceleration arrangement of a motor vehicle, as well as a control device that is designed to carry out such a method.
It is already known for hybrid vehicles or purely electrically driven vehicles to recover kinetic energy during braking processes by generator mode operation of the electrical machine and to charge a battery with said energy (regenerative braking) Here the electrical machine produces a negative drive torque, i.e. it operates as a generator. In the case in which the electrical machine is neither producing a positive nor a negative driving torque, the motor vehicle is not driven forwards and is only accelerated or decelerated because of external influences (e.g. air resistance, rolling resistance, road gradient) (said phenomenon is referred to in the English literature as “freewheeling”). In the case of negative driving torque the regenerative braking torque can be regulated as the degree of deceleration of the vehicle, which is perceptible by the driver.
In modern motor vehicles various operating concepts are known in which the electrical machine is operated as a generator. E.g. vehicles are known in which recovery is carried out in steps of fixed specified sizes. If the gas pedal is released regenerative braking is initiated for a specified drive torque (this depends on e.g. the driving mode, which e.g. can be a sports, a comfort or an environmentally friendly driving mode).
Motor vehicles are also known, in which the regenerative braking torque can be manually adjusted by the driver using switches. The regenerative braking torque is set when releasing the gas pedal (if the brake pedal is not operated). Regenerative braking is only initiated if the gas pedal is fully released. In the event of brake pedal operation the regenerative torque is additionally increased until the predefined regeneration braking torque is reached. From a specified brake pedal travel, moreover, a hydraulic brake is activated, which further increases the braking torque (additively).
Finally, motor vehicles are known, with which the regenerative braking already starts before the complete release of the gas pedal. This causes the motor vehicle to coast at a certain pedal position (i.e. the electrical machine is neither generating a positive nor a negative drive torque), and the driver can regulate the degree of the regeneration intensity by means of adjusting smaller gas pedal positions. The exact gas pedal travel at which the motor vehicle coasts can be speed-dependent here. At higher speeds said point lies at a higher gas pedal position. The reason for this is that the maximum drive torques and regeneration torques vary depending on the revolution rate and hence depending on the speed. At higher speed, frictional losses in the electrical machine have to be increasingly compensated. The consequence for the driver is that he has a longer gas pedal travel available at higher speeds for regulating the level of the regenerative braking.
Because the highest possible efficiency is to be achieved during regenerative braking and when driving the motor vehicle by means of the electrical machine, the electrical-regenerative brake is connected to one of more axles of the motor vehicle. With such an arrangement minimal friction and hence the highest possible efficiency are achieved. In other words the electrical-regenerative brake only acts upon that axle, e.g. the front axle, which is also being driven by the electrical machine. Depending on the braking situation an unbalanced and hence non-ideal distribution of the braking torques on the individual axles can occur. For example, the braking torque can act on the respective axles in a ratio that is not proportional to the respective weights on the individual axles. An unbalanced brake force distribution in turn can adversely affect the driving dynamics or the controllability of the motor vehicle. For vehicles with a regenerative deceleration arrangement, particularly strong effects on driving stability are to be reckoned with because of the desire to recover the maximum amount of braking energy. In other words there is a conflict of aims between a very high level of energy recovery by regenerative braking on the one hand and a high level of driving stability during the braking process on the other hand.